1. Field of the Invention
The present invention relates to a television tuner for receiving a television signal of a UHF band or a television signal of a VHF band and to a band switching circuit used in a television tuner.
2. Description of the Related Art
The construction of a conventional television tuner is shown in FIG. 4. A VHF input tuning circuit 33 is linked lie via a filter 32 for removing unwanted signals to an input end 31 to which a television signal of a VHF or UHF band is input. Inside the VHF input tuning circuit 33, a switch diode 33a, to the anode of which a voltage is applied, is provided, and the VHF input tuning circuit 33 is switched to tune to a high band or a low band of a VHF band in such a manner as to correspond to on/off of this switch diode. An illustration of a circuit for switching the on/off of the switch diode is omitted.
At the stage next to the VHF input tuning circuit 33, a first FET (dual-gate FET) 34 which is a constituent of a high-frequency amplifier is provided. A first gate (G1) is linked to the VHF input tuning circuit 33, and a second gate (G2) is high-frequency-grounded by a DC cut capacitor 35 and is connected to an AGC terminal 37 via a resistor 36. An AGC voltage is applied to the AGC terminal 37. Furthermore, the source (S) is grounded by a bias resistor 38 and is high-frequency-grounded by a DC cut capacitor 39. A VHF interstage tuning circuit (not shown) is connected to the drain (D), and a power voltage is supplied to the drain via the VHF interstage tuning circuit. Furthermore, the collector of a switch transistor 40 is connected to the first gate via a resistor 41, and the collector is connected to a power terminal 43 via a resistor 42. A power voltage is applied to the power terminal 43. The emitter is grounded.
The anode of a switch diode 44 is linked to the filter 32. A voltage from the VHF input tuning circuit 33 is supplied to the anode of the switch diode 44, and a UHF input tuning circuit 45 is linked to the cathode. The cathode of the switch diode 44 is connected to the collector of the switch transistor 40 via a resistor 46.
At the stage next to the UHF input tuning circuit 45, a second FET (dual-gate FET) 47 which is a constituent of a high-frequency amplifier is provided. The first gate of the second FET 47 is linked to the UHF input tuning circuit 45. One end of a peaking coil 48 is connected to the first gate, and the other end thereof is high-frequency-grounded by a DC cut capacitor 49. The connection point of the peaking coil 48 and the DC cut capacitor 49 is connected to a switch terminal 50 and is connected to the base of the switch transistor 40 via a resistor 51. Furthermore, the second gate is high-frequency-grounded by a DC cut capacitor 52 and is connected to the AGC terminal 37 via a resistor 53. Furthermore, the source is grounded by a bias resistor 54 and is high-frequency-grounded by a DC cut capacitor 55. A UHF interstage tuning circuit (not shown) is connected to the drain, and a power voltage is supplied to the drain via the UHF interstage tuning circuit.
In the above construction, when a UHF-band television signal is to be received, a high-level switching voltage is applied to the switch terminal 50. Just then, a predetermined bias voltage required for operation is applied to the first gate of the second FET 47, causing the second FET 47 to be placed in an operating state. At the same time, the switch transistor 40 is turned on, thereby the switch diode 44 is turned on, and thus it becomes possible for a television signal to be input to the UHF input tuning circuit 45. At this time, the voltage of the first gate of the first FET 34 becomes a bias voltage of 0 volts, and operation becomes impossible.
When a television signal of a VHF band is to be received, the switch terminal 50 reaches a low level. Therefore, the voltage of the second gate of the second FET 47 becomes a bias voltage of 0 volts, and operation becomes impossible, thereby the switch transistor 40 is turned off. For this reason, the switch diode 44 is also turned off, and thus a television signal is not input to the UHF input tuning circuit 45. Furthermore, since the switch transistor 40 is off, a predetermined bias voltage required for operation is applied to the first gate of the first FET 34.
A band switching circuit of a conventional television tuner is shown in FIG. 5. A VHF antenna tuning circuit 132 connected to an antenna terminal 131 is switched to tune to a high-band television signal or a low-band television signal of a VHF band according to on/off of a switch diode 132a provided in the VHF antenna tuning circuit.
At the stage next to the VHF antenna tuning circuit 132, a VHF amplifier 133 is provided, and at the stage next to the VHF amplifier 133, a VHF interstage tuning circuit 134 is provided.
The VHF interstage tuning circuit 134 is formed of a double tuning circuit, with a switch diode 134a being provided therein. Similarly to the VHF antenna tuning circuit 132, the VHF interstage tuning circuit 134 is switched to tune to a high-band television signal or a low-band television signal of a VHF band according to on/off of the switch diode 134a. 
The cathode of the switch diode 132a in the VHF antenna tuning circuit 132 and the cathode of the switch diode 134a in the VHF interstage tuning circuit 134 are DC-grounded by means of a resistor (not shown).
At the stage subsequent to the VHF interstage tuning circuit 134, a VHF mixer (not shown), etc., are provided. The VHF antenna tuning circuit 132, the VHF amplifier 133, the VHF interstage tuning circuit 134, the VHF mixer, etc., constitute the VHF tuner section.
Furthermore, a UHF antenna tuning circuit 135 is connected to the antenna terminal 131, and at the stage next to the UHF antenna tuning circuit 135, a UHF amplifier 136 is provided. The UHF amplifier 136 is switched to an operating state only when a UHF-band television signal is to be received. At the stage next to the UHF amplifier 136, a UHF interstage tuning circuit 137 is provided. The UHF interstage tuning circuit 137 is also formed of a double tuning circuit. At the stage subsequent to the UHF interstage tuning circuit 137, a UHF mixer (not shown), etc., are provided. The UHF antenna tuning circuit 135, the UHF amplifier 136, the UHF interstage tuning circuit 137, the UHF mixer, etc., constitute the UHF tuner section.
The switching of the tuning frequency band between the VHF antenna tuning circuit 132 and the VHF interstage tuning circuit 134, and the operation switching of the UHF amplifier are performed by a switching voltage generation circuit 141. The switching voltage generation circuit 141 is formed within an integrated circuit 140. Inside the integrated circuit 140, in addition to the switching voltage generation circuit 141, an oscillation circuit, a PLL circuit, etc., are formed. However, illustrations of them are omitted.
The integrated circuit 140 is provided with at least four terminals 140a, 140b, 140c, and 140d related to the switching voltage generation circuit 141. Also, the switching voltage generation circuit 141 is provided with at least three independent switch transistors 141a, 141b, and 141c. The collector of each of the switch transistors 141a, 141b, and 141c is connected to the first terminal 140a. A power voltage is supplied to the first terminal 140a. 
Furthermore, the emitter of the first switch transistor 141a is connected to the second terminal 140b, the emitter of the second switch transistor 141b is connected to the third terminal 140c, and the emitter of the third switch transistor 141c is connected to the fourth terminal 140d. 
In addition, the second terminal 140b is DC-connected to the cathode of the switch diode 132a in the VHF antenna tuning circuit 132 and to the cathode of the switch diode 134a in the VHF interstage tuning circuit 134. The third terminal 140c is DC-connected to the anode of the switch diode 132a in the VHF antenna tuning circuit 132 and to the anode of the switch diode 134a in the VHF interstage tuning circuit 134. The fourth terminal 140d is connected to the bias voltage supply end of the UHF amplifier 136. Outside the integrated circuit 140, the series circuit of a resistor 142 and a diode 143 allows the third terminal 140c and the fourth terminal 140d to be connected to each other. The diode 143 is connected in an orientation such that the anode thereof is on the side of the fourth terminal 140d and the cathode thereof is on the side of the third terminal 140c. 
A high-level or low-level control voltage for independently turning on/off each of the switch transistors 141a, 141b, and 141c is applied to their respective bases. Whether the control voltage becomes a high level or a low level is determined by a channel selection signal input to the integrated circuit 140. Each switch transistor is turned on in accordance with the high-level control voltage and is turned off in accordance with the low-level control voltage.
FIG. 6 shows the on/off state of each of the switch transistors 141a, 141b, and 141c when switching is made to a target band. In FIG. 6, reference character TR1 denotes a first switch transistor 141a. Reference character TR2 denotes a second switch transistor 141b. Reference character TR3 denotes a third switch transistor 141c. 
First, when a UHF-band television signal is to be received, the first and second switch transistors 141a and 141b are turned off, and the third switch transistor 141c is turned on. Just then, a power voltage appears at the fourth terminal 140d, and this voltage is applied as a switching voltage to the UHF amplifier 136, causing the UHF amplifier 136 to be placed in an operating state. At this time, since a switching voltage is supplied to the anodes of the switch diodes 132a and 134a via the resistor 142 and the diode 143, these switch diodes are turned on, so that the VHF antenna tuning circuit 132 and the VHF interstage tuning circuit 134 are switched to tune to a high band of the VHF band. This avoids interference from each of the tuning circuits 132 and 134.
When a television signal of a high band of the VHF band is to be received, the first switch transistor 141a is turned off, the second switch transistor 141b is turned on, and the third switch transistor 141c is turned off. Just then, a power voltage appears at the third terminal 140c. This voltage is supplied as a switching voltage to the anode of each of the switch diodes 132a and 134a, these diodes are turned on, and each of the tuning circuits 132 and 134 tunes to the high band. At this time, the supply of the switching voltage to the UHF amplifier 136 is stopped by the diode 143.
Next, when a television signal of a low band of the VHF band is to be received, the first switch transistor 141a is turned on, and the second switch transistor 141b and the third switch transistor 141c are turned off. Just then, a power voltage appears at the second terminal 140b, this voltage is supplied as a switching voltage to the cathode of each of the switch diodes 132a and 134a, these diodes are turned off, and the two tuning circuits 132 and 134 tune to the low band.
As described above, the operation switching of the first FET 34 and the switch diode 44 is performed by the switch transistor 40. However, since individual parts are used for the first FET 34 and the second FET 47, problems arise in that wiring for an interconnection of the switch transistor 40 and each of the first FET 34 and the second FET 47 becomes complex, and the space factor when each part is disposed on a board is decreased.